Paulina Vera, a staff attorney with CASA de Maryland, works with illegal immigrants on Jan. 12, following a seminar. A new study says the nation’s illegal immigrant population has fallen below 11 million, its lowest level in more than a decade. (Evelyn Hockstein/For The Washington Post)

The illegal immigrant population in the United States has fallen below 11 million, continuing a nearly decade-long decline that has the potential to reshape the debate over reforming the nation’s immigration system, according to a study released Wednesday.

The total undocumented immigrant population of 10.9 million is the lowest since 2003, says the report from the Center for Migration Studies, a New York think tank. The number of undocumented immigrants has fallen each year since 2008, the report says, driven primarily by a steady decline in illegal migrants from Mexico. Sharper declines from South America and Europe have contributed to the overall numbers, the report says, even as illegal immigration from Central America — where families with children have flocked across the southwest border in recent months — is on the rise.

With its release as voting nears in the 2016 presidential primaries, the 15-page document could impact the fiery debate over immigration unfolding on the campaign trail. Republican candidates, led by Donald Trump, have portrayed the border as overwhelmed by illegal immigrants who must be kept out by a massive wall the New York developer proposes to build. President Obama and Democratic candidates say the border has never been more secure and call for comprehensive immigration reform to naturalize immigrants already here.

[interstitial_link url="https://www.washingtonpost.com/politics/flow-of-illegal-immigration-slows-as-us-mexico-border-dynamics-evolve/2015/05/27/c5caf02c-006b-11e5-833c-a2de05b6b2a4_story.html"]Fewer immigrants are entering the U.S. illegally, and that’s changing the border security debate[/interstitial_link]

While the report — written by Robert Warren, a former longtime U.S. government demographer — is an estimate based on 2014 U.S. census data and doesn’t take a position on immigration, officials from the center took issue with the characterizations of Republican candidates. “The facts of the report tell a different story than what you might hear on the campaign trail or in the halls of Congress, where many send a message that we’re being overrun by undocumented immigrants,” Kevin Appleby, the center’s senior director of international migration policy, said on a press call after the report’s release. “The facts and the data show that’s just not true. Hopefully, political discourse will be more fact-based going forward.”

Federal authorities launched a nationwide operation to remove Central American illegal immigrants from the U.S. Here is what you need to know about why agents are targeting this group. (Claritza Jimenez/The Washington Post)

The debate over how to handle illegal migrants already here has escalated even more in recent weeks. The Obama administration has launched a series of raids aimed at deporting the mostly women and children who have come from Honduras, Guatemala and other Central American countries since 2014. Administration officials are now working frantically to quell political outrage the raids have triggered among immigration rights advocates and Latino leaders.

[interstitial_link url="https://www.washingtonpost.com/politics/obama-struggling-with-immigration-rules-and-cruelties-of-deportation/2016/01/18/5c2d4258-bba7-11e5-b682-4bb4dd403c7d_story.html"]Obama struggling with immigration rules and cruelties of deportation[/interstitial_link]

In some ways, the new report echoes other research showing that the political debate may have already been overtaken by changing immigration patterns around the nation. Demographers at Pew Research Center, for example, found last year that the number of illegal immigrants — which more than tripled, to 12.2 million, between 1990 and 2007 — had since dropped by about 1 million. Pew’s preliminary estimate counted the total illegal immigration population at 11.3 million, as of 2014.

The new Center for Migration Studies report is the first in recent years to peg the number as falling below 11 million, giving it at least a symbolic significance, since the 11 million number has been widely used by politicians and journalists.

Although the new report does not cite specific reasons for the decline, other experts have attributed it to a combination of tighter U.S. border security measures and economic and demographic changes in Mexico, such as women having fewer children. The altered dynamics have changed the makeup of the undocumented population. Until recent years, illegal immigrants tended to be young men streaming across the Southern border in pursuit of work. But demographic data show that the typical illegal immigrant now is much more likely someone who is 35 or older and has lived in the United States for a decade or more.

The report reveals a series of other nuggets about an undocumented immigrant population that researchers have struggled to understand, since some still live in the proverbial shadows. The decline in illegal immigrants was driven primarily by nine percent drop in those arriving from Mexico since 2010, it said, though six million of the 10.9 million total are still originally from Mexico. During the same period, illegal immigrants arriving from South America fell 22 percent and those coming from Europe declined 18 percent.

At the same time, however, the undocumented population from Central America — where the surge of families and unaccompanied minors since 2014 has originated — went up five percent, the report said.

Only Texas and Virginia, of the 10 states with the highest undocumented populations, gained undocumented residents from 2010 to 2014, the report said. All others saw their undocumented populations decline, including California, which the report said leads the nation with 2.6 million people here illegally.

Demographers at the center also found that state laws designed to restrict the numbers of undocumented immigrants, such as the well known one passed in Arizona, “did not have a lasting impact” on the size of the undocumented population.

The report’s release came as the Department of Homeland Security, which enforces immigration laws, reported that more than 525,000 people overstayed their visas in fiscal year 2014 and that about 482,000 of them are suspected to be in the country illegally.

But the DHS report examined nearly 45 million non-immigrant visas, meaning that 98.8 percent of the visa-holders left the country on time, the department said in a statement. Center for Migration Studies officials said they did not believe the overstay numbers would affect their overall estimates of how many people are in the country illegally.

Warren, the report’s author, who worked as a government demographer for more than 30 years, said the very low overstay rate was similar to what it was as far back as the 1980s and apparently has not increased over time. He added that the 525,000 people who stayed beyond their visas last year would likely be counteracted by “a lot of people who came in previous years, overstayed their visas and would have left last year.”

Jerry Markon covers the Department of Homeland Security for the Post’s National Desk. He also serves as lead Web and newspaper writer for major breaking national news.